We have previously identified several biological systems including neutrophil activation, aging and gliosis in which proteins are oxidatively modified in reactions similar to the MFO-mediated reactions we have characterized in vitro. In the past year, we have focused on the regulation of respiratory burst activity in an effort to understand the process of neutrophil activation and the mechanisms of protein modification. We have partially characterized a novel reaction in which tyrosine is incorporated into neutrophil proteins by a mechanism which is dependent on PMA-stimulated respiratory burst activity and independent of protein synthesis. At least one product of the reaction appears to be similar if not identical to dityrosine. However, synthesis of the authentic product is required for verification. We have also investigated the possibility that activation of G-6-PD during neutrophil activation may be due to phosphorylation mediated by protein kinase C. However, no phosphorylation of the enzyme has been detected under a variety of conditions. Isoelectic focusing experiments revealed that multiple forms of G-6-PD are present in G-6-PD purified from control or from activated cells. These results suggest that increased activity and increased heat stability of the enzyme from activated cells may be due to limited proteolysis. Finally, in the gliosis model system, treatment of injured spheres with Alpha-tocopherol depresses gliotic index, malondialdehyde formation and protein oxidation using DNPH-reactivity. Moreover, the effects of mechanical injury in this system can be mimicked by treatment with Fe/ADP/oxygen. Again Alpha-tocopherol depresses both gliotic index and malondialdehyde formation. These results suggest that neuronal damage in gliosis may be due in part to MFO reactions.